Heat treatment of metal



p 30, 1969 w. MAKNIS 3,470,035

HEAT TREATMENT OF METAL Filed Jan. 13, 1965 United States Patent 3,470,035 HEAT TREATMENT OF METAL Walter Maknis, W. Main St., Woodbury, Conn.

Filed Jan. 13, 1965, Ser. No. 425,198 Int. Cl. C22f 1/00; C2111 1/00 US. Cl. 14813 9 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to the heat treatment of metal articles and more particularly to a heat treating process and to a metallic container for use in such process.

Many metal parts are heat treated during manufacture to develop desirable properties, such as surface hardness.

Because of the elevated temperatures necessary in such heat-treating processes, metals heat treated in normal room atmosphere react chemically with the oxygen and other gases and vapors in the air and develop undesirable surface conditions, e.g., oxide scale, loss of desirable brightness, or loss of surface carbon. Expensive metal removal operations must then be performed to eliminate the undesirable surface condition. To overcome the need for such expensive metal removal operations, various special heat-treating furnaces, retorts and processes have been devised whereby the workpieces may be surrounded during heat treatment by a protective atmosphere or may be heat-treated in a liquid bath, or in a substantial vacuum. Such heat-treating equipment and processes heretofore known have been complicated and expensive. Atmosphere furnaces and salt bath furnaces are generally high production type units, the cost of which cannot be justified by the small or intermittent heat treater, and vacuum furnace equipment is even more expensive. The need for a quenching operation, i.e., fast cooling immediately following the heat treatment, which is essential with some heat treating operation-s, constitutes a further obstacle to effective surface protection of the workpiece in many processes. Processes including heat treatment in a special atmosphere or in a substantial vacuum in special retorts, including light gauge, bag type metal retorts, as in Patents 3,092,524 and 3,130,089 require, in addition to the basic furnace, at least special pumping equipment or an atmosphere generator or other source of atmosphere, or other auxiliary equipment which are initially costly and/ or are expensive to operate.

The present invention has as an object to overcome the foregoing disadvantages of heretofore-known heat treating processes and equipment and to provide a process, and a protective container for use therein, whereby high quality heat treatment operations can be effected with the use of only the simplest type of furnace and heating equipment, eliminating the necessity for high capital investment.

Another object of the invention is to provide a heat treating process within the reach of the small or intermittent user.

Another object is to provide a simple and inexpensive heat-treating process wherein objectionable surface deterioration of the workpieces from contact with air can be effectively inhibited.

A further object of the invention is to provide a simplified protective covering for protecting the surface of the metal workpiece from surface deterioration during heat treatment operations.

Other and further objects, features and advantages of the invention will become apparent from the following description of illustrative embodiments and procedures thereof, in which description reference is made to the accompanying drawing, wherein FIG. 1 is a perspective view, somewhat schematic, illustrating the insertion of a workpiece in the protective covering of the invention;

FIG. 2 is an elevation of the covering with the workpiece therein in condition for insertion into the heattreating furnace;

FIG. 3 is a cross-sectional view of the workpiece and covering of FIG. 2 illustrating a condition during initial heating in the furnace.

FIG. 4 is similar to FIG. 3 but illustrating a condition subsequent to the condition of FIG. 3.

FIG. 5 is a schematic view illustrating different kinds of quenching operations following the heat treatment procedure;

FIG. 6 is a perspective view illustrating a procedure for preventing welding of the covering to the work piece; and

FIG. 7 is an elevation of a workpiece, illustrating a further method of preventing welding of the covering to the workpiece.

In accordance with the invention I have found that objectionable surface deterioration of metal workpieces which are subject to surface deterioration fromcontact with air at elevated, heat-treating temperatures can be effectiyely inhibited during heat treatment operations by first enclosing the workpiece in a covering of a thin, flexible, oxidizable-metal sheet completely enclosing the workpiece but without first exhausting or evacuating the air from the covering or displacing the air with an inert atmosphere. Typically the covering is a metal bag, open along one edge only, into which the workpiece is inserted, whereafter the open edge is closed in any suitable way, as by folding it upon itself and compressing the folds in the manner of a double seam. This closure need not be an hermetic one. Before effecting this closure, some of the air can be squeezed from the covering by simply compressing it by hand. No exhausting or evacu' ating operation is performed on the covering or bag. The covering and workpiece is then subjected to the heat treatment operation at elevated temperature. With this procedure, the heat-treating furnace or other heating means may be of the simplest and most inexpensive sort.

Referring now to the drawing, for purposes of illustration the article to be heat treated is shown as a cylindrical metal workpiece 2. The covering is in the form of a bag 4 formed from two sheets 5, 5' of ferrous metal, hermetically sealed along three edges as by lines of electric resistance welding at 6, 8 and 10. These edges may be sealed by any other appropriate means, such as any conventional sheet metal joint. The other edge 12 of the bag 4 is left open. Such bags have the great advantage that they can be inexpensively manufactured as they are of very simple construction and do not have special fittings or connections for connecting the bag to pumping equipment for exhausting the bag or for flooding it with a special atmosphere.

As shown in FIG. 1, the workpiece 2 is inserted into the bag 4 through the open end 12. The bag 4 then may be pressed on its outside by hand to press out a greater or lesser amount of the air from the bag. The open end 12 is then closed, as by folding it upon itself and again folding the folded portion and compressing it to form a double seam at 14.

The closed bag 4 containing the workpiece and some remaining air at atmospheric pressure is now placed in a heat-treating furnace and subjected to the heat treating operation at elevated temperature. As the walls 5, of the bag 4 are thin they are heated very rapidly, more rapidly than the workpiece 2 itself, so that the inner surface of the bag reaches elevated temperatures more quickly than the surface of the workpiece. Heat is transmitted from the inner surface of the walls 5, 5' of the bag to the air within the bag and I have found that the bag first tends to swell as the air within it expands due to the increase in the air temperature. In some cases the expansion of the air within the bag 4 and consequent puffing of the bag during the initial heating may move the walls 5, 5' of the bag largely out of contact with the workpiece 2, into the condition illustrated in FIG. 3, thereby desirably lessening heat transfer from the furnace to the workpiece during this initial stage. In any case, the interior surface of the bag 4 is heated in the furnace more rapidly than is the surface of the workpiece 2 itself. Reaction of components of the enclosed air with the interior surface of the bag 4 is promoted at such elevated temperatures and it appears that the capacity of the contained air to produce undesirable surface effects is rapidly reduced by reaction with or adsorption or absorption on the interior surface of the bag 4 and is largely or entirely spent before the surface of the workpiece 2 can come up to sufficiently high temperatures to bring about significant deleterious effect on its surface. This action is confirmed by observation of'the fact that after the initial pufiing of the bag 4, the bag, in some cases at higher heat treating temperatures, then collapses and may reach or approach the condition illustrated in FIG. 4 in which it closely hugs the workpiece 2.

Continued heating of the covering and workpiece in the furnace brings the workpiece up to heat-treating temperature and effects the desired heat treatment. It is believed that combination with the bag of the oxygen, other gases and vapors entrapped in the bag proceeds to such an extent before the workpiece 2 itself comes up to high temperature that the workpiece is left in an essentially neutral atmosphere by the time it has reached a temperature sufficiently elevated to bring about undesirable surface effects from the atmosphere in the bag. The large interior surface of the bag, as compared with the surface area of the metal workpiece, promotes this favorable action. In effect, during the heat treatment which occurs after the workpiece has reached heat treating temperature, the bag functions to provide a substantially gastight enclosure that protects the workpiece from undesirable surface deterioration during the heat treating operation.

After the heat-treatment is complete the bag still continues to provide a substantially gas-tight enclosure containing a neutral atmosphere to protect the workpiece during cooling. When quenching is required, advantage can be taken of this condition to protect the workpiece 2 by maintaining it enclosed in the bag in a protected state. Under some conditions it may be preferred to immerse the closed bag containing the workpiece directly in the quenching medium 16 as illustrated at 4 in FIG. 5. If the workpiece is quenched while sealed in the bag, it may be desirable to use a faster quenching medium than would be employed if the workpiece were quenched in direct contact with the medium, as the bag will impede heat transfer between the medium and the workpiece. For example, when quenching in the bag, oil may be employed instead of air, or water instead of oil, or a brine solution instead of water, or other changes made in the quenching routine as will be readily understood and apparent to those skilled in the art. In other cases it may be preferred to remove the bag before quenching. For example, as illustrated in FIG. 5, the sealed bag 4 can be held directly over the quenching medium 16 and an end of the bag cut off with conventional scissors 18, or other appropriate cutting means, to allow the workpiece 2 to fall immediately into the quenching medium, with a minimum of exposure to the air.

The collapse of the bag during heat treatment is a function of time and temperature, with little or no collapse at 1500 F. and relatively complete collapse at 2200 F. in the time required for a heat treatment. If the bag is tightly sealed and if a large proportion or all of the air has been squeezed out of it before it was sealed, the collapse of the bag during heat treatment at temperatures in the neighborhood of 2200 F. may be so complete that a surface of the bag will weld to itself and to a tool of high speed steel contained in it. At temperatures in the neighborhood of 1800" F. the rate of collapse is much slower and welding does not take place.

Such objectionable welding of the bag to the workpiece can generally be avoided by partly or entirely omitting the step of squeezing the bag to eject air from it before it is sealed, thus leaving a larger amount of air in the bag so that an objectionable degree of collapse is not reached in the length of time required for the heat treatment. I have obtained extremely fine hardening of a high-speed-steel tool 20, FIG. 7, by first wrapping it with a chromium nickel wire 22 arranged in spaced coils before inserting it in the bag 4. I have also obtained excellent results by utilizing a relatively deep bag, in which the workpiece is placed close to the bottom, by leaving the open edge 12 of the bag unsealed, and placing a metal wire 24 so that its inner end 26 lies in contact with the workpiece 2, FIG. 6, and its outer end 28 extends outside the bag through the open edge 12, the open end 12 of the bag being left largely unsealed, being closed merely by a simple corner fold, as shown in FIG. 6. The wire serves to maintain a vent for the bag to prevent it from collapsing to an undesired extent. As the gases in the bag react or absorb, additional gas may be drawn in through the vent passage. Due to the additional length of the bag, any such additional gas is combined or reacted with the bag before it reaches the workpiece. At the end of the heat-treatment-step the wire can be removed from the bag and the bag can be sealed by folding it before it is removed from the furnace. Thereafter the quenching operation and subsequent operations can be performed as before. For some types of workpieces it may not be necessary to remove the Wire before quenching, as any additional air drawn into the bag through the vent passage will not cause objectionable surface effects on the workpiece. Excellent results in preventing fusion of the bag to the workpiece also have been obtained by first applying to the surface of the workpiece, before placing it in the metallic covering, particles of a refractory powder, for example a pow dered mortar, cement or other refractory powder.

As the material for the bag 4, I may use various ferrous metals which are sufiiciently oxidizable to accomplish the desired purpose, such as various steels and stainless steels. American Iron and Steel Institute type 309 stainless steel, 0.002 inch thick has been found highly effective for use with high speed type tool steels at temperatures up to 2225 F. The sheet metal of the bag should be of light gauge to promote rapid heating and provide the desired flexibility but sufiiciently strong to withstand the necessary processing. Thicknesses from 0.001" to 0.012" are satisfactory.

The following examples of particular applications of the invention are given by way of illustration and not of limitation.

Example I A block formed from American Iron and Steel Institute (A.I.S.I.) type D-2 (high carbon, high chrome type steel) was placed in a bag approximately 4 inches by 6 inches consisting of two sheets of A.I.S.I. type 309 stainless steel 0.002" thick hermetically sealed together along three edges by electrical resistance welding as in FIG. 1. The open end of the bag was closed by double folding the edges of the opening on the fourth side and pressing them together. The bag with the enclosed block was then placed in a heat treating furnace and heat treated at approximately 1850 F. for 32 minutes. The bag was removed from the furnace and, while still sealed, was immersed in an oil quenching medium to quench the block. Upon removal from the bag, the block was found to be desirably heat-treated and to have a surface which was bright and free from scale and other objectionable surface deterioration. The results were considered excellent.

Example II A sample block of A.I.S.I. type A-2 air hardening steel was wrapped in a sheet of A.I.S.I. type 309 stainless steel 0.002" thick by rolling the block in the sheet in the air to completely cover the block and then folding the ends of the sheet upon themselves, but without sealing the edges to themselves or exhausting air from the wrapping. The wrapped block was placed in a heat-treating furnace and hardened at 1750" F. for 25 minutes. The wrapped block was then removed from the furnace and allowed to cool in the air without disturbing the wrapping. Upon removing the wrapping it was found that the sample was excellently hardened and showed only slight surface discoloration, which would be unobjectionable for most purposes.

Example III A punch 2 A" in overall diameter, reduced to 1%" diameter by 2 /2 long, was wrapped in a stainless steel sheet as in Example II. The wrapped punch was hardened for 1- /2 hours at 1550 F., removed from the furnace and quenched by blowing air over the wrapping with an electric fan. When unwrapped the punch showed a Rockwell C hardness of 62 and a gun metal blue finish. The results were considered very good.

Example IV A sample block of a high speed steel, A.I.S.I. type M-l, was wrapped in a stainless steel sheet as in Example II and heated in a furnace to 1600 F. and allowed to cool with the furnace, according to a standard annealing procedure. Good results were obtained.

Example V A tap made from A.I.S.I. type M-1 high speed steel was wrapped with nickel chromium wire as in FIG. 7 and was placed in a stainless steel bag as in Example I. The bag was placed in a simple furnace and hardened for 12 minutes at 2200 F. and then oil quenched. The tap was readily removable from the bag and was hardened with excellent results.

Example VI A tap of high speed steel A.I.S.I. type M-l was placed in a stainless steel bag of A.I.S.I. type 309 stainless steel 0.002" thick and 12" deep from the open end to the opposite closed end. The tap was placed in the bag as shown in FIG. 6 with the wire 24 in contact with the tap and leading out of a corner of the bag, the bag being folded diagonally as shown. The bag was then hardened for 12 minutes at 2200 F. The bag was removed from the furnace and with the wire still in place was immersed in an oil quenching medium. The bag was readily removed from the tap. The results were considered very good with only slight discoloration, evidently resulting from air drawn into the bag during quenching.

Example VII A tap as in Example VI was first dipped in a powdered high temperature cement (Robinson Crown Dry Bonding Mortar, manufactured by Robinson Clay Products Co., Akron, Ohio). The tap with powder clinging to it was then enclosed in a bag as in Example I, and hardened for 12. minutes at 2200 F. The bag was then removed from the furnace and immersed in oil to quench the tap. Upon removal of the bag it was found that the tap was readily separated from the bag and cement and the results were considered excellent.

It will be seen from the foregoing that the invention has provided a protective covering and process for protecting metal workpieces from oxidative or other surface deterioration during heat treatment which are very simple yet highly effective for the intended purpose. The process may be practiced with heat treating furnaces of the simplest and least expensive type as it does not require the use of special atmospheres or salt baths or auxiliary pumping or atmosphere generating equipment or other source of atmosphere, and, accordingly, the invention greatly reduces the required capital expenditure of the heat treater, thus bringing heat treatment within the reach of smaller production applications. No exhausting or evacuating operation is required and the bag of the invention does not require any special fittings or connections for pumping equipment.

I claim:

1. The method of heat treating a metal article which is subject to objectionable surface deterioration from contact with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises enclosing the article in the air in a covering of a thin, flexible oxdizable-metal sheet completely enclosing said article and without including in the covering any separate oxygen-removing material, thereafter heat treating the article by subjecting the article and covering in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the covering and so as to substantially inhibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and while said covering contains air at atmospheric pressure, and thereafter removing the covering from the article.

2. The method of heat treating a metal article which is subject to objectionable surface deterioration from contact with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises enclosing the article in the air in a closed bag of a thin, flexible oxidizable-metal sheet completely enclosing said article and without including in the bag any separate oxygen-removing material, thereafter heat treating the article by subjecting the article and bag in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the bag and so as to substantially inhibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and while said bag contains air at atmospheric pressure, and thereafter removing the bag from the article.

3. The method of heat treating a steel article which is subject to objectionable surface deterioration from contat with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises enclosing the article in the air in a closed bag of a thin, flexible steel sheet completely enclosing said article and without including in the bag any separate oxygen-removing material, thereafter heat treating the article by subjecting the article and bag in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperawithin the bag and so as to substantially inhibit said atture without previously having exhausted the air from mosphere at the heat treating temperature from reaching said article in its deleterious form, and while said bag contains air at atmospheric pressure, and thereafter removing the bag from the article.

4. The method of heat treating a steel article which is subject to objectionable surface deterioration from contact with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises enclosing the article in the air in a closed bag of a thin, flexible stainless steel sheet completely enclosing said article and without including in the bag any separate oxygen-removing material, thereafter heat treating the article by subjecting the article and bag in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the bag and so as to substantially inhibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and while said bag contains air at atmospheric pressure, and thereafter removing the bag from the article.

5. The method of heat treating a metal article which is subject to objectionable surface deterioration from contact with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises wrapping a metal wire about the article, enclosing the so-wrapped article in the air in a covering of a thin flexible oxidizablemetal sheet completely enclosing said article and without including in the covering any separate oxygen-removing material, thereafter heat treating the article by subjecting the wrapped article and covering in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the covering and so as to substantially inhibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and while said covering contains air at atmospheric pressure, and thereafter removing the covering and wire from the article.

6. The method of heat treating a metal article which is subject to objectionable surface deterioration from contact with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises enclosing the article in the air in a bag of a thin, flexible oxidizable-metal sheet open along one edge only and without including in the bag any separate oxygen-removing material, placing a metal wire in said bag with one end adjacent said article and its other end extending outside said bag through said open edge, closing said open edge about said wire, thereafter heat treating the article by subjecting the article, wire and bag in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the bag and so as to substantially in hibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and while said bag contains air at atmospheric pressure, and thereafter removing the bag and wire from the article.

7. The method of heat treating a metal article which is subject to objectionable surface deterioration from contact with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises enclosing the article in the air in a bag of a thin, flexible oxidizablemetal sheet open along one edge only, pressing the bag about the article to reduce the air content of the bag, closing said open edge of the bag and without including in the bag any separate oxygen-removing material, thereafter heat treating the article by subjecting the article and bag in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the bag and so as to substantially inhibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and While said bag contains air at atmospheric pressure, and thereafter removing the bag from the article.

8. The method of heat treating a metal article which is subject to objectionable surface deterioration from contact with air at elevated, heat-treating temperatures, and of inhibiting such deterioration, which comprises enclosing the article in the air in a covering of a thin, flexible oxidizable-metal sheet completely enclosing said article and without including in the covering any separate oxygen-removing material, thereafter heat treating the article by subjecting the article and covering in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the covering and so as to substantially inhibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and while said covering contains air at atmospheric pressure,

quenching the article while it is enclosed in said covering, and thereafter removing the covering from the article.

9. The method of heat treating a metal article which is subject to objectionable surface deterioration from contact with air at elevated, heat treating temperatures, and

of inhibiting such deterioration, which comprises applying to the surface of the article particles of a refractory powder, enclosing the article in the air in a covering of a thin, flexible, oxidizable-metal sheet completely enclosing said article with said particles adhering thereto to prevent adherence of the article to said covering, and without including in the covering any separate oxygen-removing material, thereafter heat treating the article by subjecting the article and covering in an atmosphere deleterious to the surface of the article to a heat-treating elevated temperature without previously having exhausted the air from within the covering and so as to substantially inhibit said atmosphere at the heat treating temperature from reaching said article in its deleterious form, and

while said covering contains air at atmospheric pressure, and thereafter removing the covering from the article.

References Cited UNITED STATES PATENTS US. Cl. X.R. 

